1. Field of the Invention
The present invention relates to an apparatus and method for producing blown tubular films from thermoplastic resinous materials. More particularly, this invention relates to improved tube forming and cooling procedures wherein a blown tube is formed by the entrapped bubble method and is cooled and stabilized by an adjustable stabilizer air ring as it is being advanced to further processing.
2. Description of the Prior Art
Generally, the prior art techniques for forming tubular thermoplastic films comprise continuously extruding a melt of a thermoplastic material through an annular orifice, applying internal air pressure to the tube thus formed and shape expanding the tube and reducing the wall thickness thereof to appropriate dimensions while cooling and solidifying the extruded thermoplastic. Thereafter, the formed tubing is collapsed by passing it through the nip formed by a pair of counter-rotating pinch rolls. The flattened tubing may be subsequently passed to a wind up station or on to further processing such as bag-making operations for example.
Although useful tubing has been commercially prepared utilizing this method, when production rates are increased, i.e. when faster rates of tubular advancement are desired, there is a tendency for the advancing bubble before it is collapsed, to becme unstable and begin to flutter while the tube is till in a semi-motion condition. This will result in bubble failure or rupture and cause an attendant shutdown of the production line. Accordingly, one of the major problems in this area is to rapidly cool the extrudate bubble of thermoplastic material in order to maintain optimally high production rates. Conventionally, this can be accomplished utilizing air rings which are positioned around the external circumference of the tube as it is being extruded. In general, such air rings comprise a circular orifice through which air is fed under pressure from a plenum chamber. The cooling air which is directed at the tube helps to solidify the molten polymer. Also in the past, to assist in maintaining bubble stability downwstream from the cooling air ring, which air ring is usually positioned immediately adjacent to the tubular die, stabilizer rings, comprising a ring element through which the tube is passed, are positioned around the tube. These stabilizing rings, to some degree, reduce bubble instability at high production rates. Obviously, however, when it is desired to change the diameter of the extrudate tube to meet particular end use requirements, the extrusion line must be shutdown so that the stabilizer ring may be replaced with another ring having the desired diameter.
Applicant has now found that it is possible to combine the features of the stabilizing element with additional cooling means to achieve increased production efficiencies. Additionally applicant has devised an apparatus which may reamin in-line during change in the size of the extruded tubing thereby eliminating the necessity for line shutdowns required in the past.